[swift-evolution] [Pitch] Requiring proactive overrides for default protocol implementations.

Tod Cunningham tcunningham at vectorform.com
Wed Apr 27 15:17:33 CDT 2016

Having to specify override to override the implementation of protocol leads one further down the path that the protocol’s implementation is actually being overridden.  However, currently that is not the case.  If the protocol’s definition was actually being overridden, I think you would have a good case.  But swift, as it exists today, will still call the protocol’s default implementation if accessed through the protocol type.  So it isn’t really overriding the implementation of the protocol:

For example:

protocol Test {


extension Test {

    var hello: String {

        return "hello"



class A: Test {


class B: Test {

    var hello: String {

        return "see ya"



print( A().hello ) // hello

print( B().hello ) // see ya

let test1: Test = A()

print( test1.hello )  // hello

let test2: Test = B()

print( test2.hello ) // hello  <== not "see ya"


Tod Cunningham

From: <swift-evolution-bounces at swift.org<mailto:swift-evolution-bounces at swift.org>> on behalf of Erica Sadun via swift-evolution <swift-evolution at swift.org<mailto:swift-evolution at swift.org>>
Reply-To: Erica Sadun <erica at ericasadun.com<mailto:erica at ericasadun.com>>
Date: Wednesday, April 27, 2016 at 1:10 PM
To: swift-evolution <swift-evolution at swift.org<mailto:swift-evolution at swift.org>>, Jordan Rose <jordan_rose at apple.com<mailto:jordan_rose at apple.com>>, Douglas Gregor <dgregor at apple.com<mailto:dgregor at apple.com>>
Subject: [swift-evolution] [Pitch] Requiring proactive overrides for default protocol implementations.

From the Swift Programming Language: Methods on a subclass that override the superclass’s implementation are marked with override—overriding a method by accident, without override, is detected by the compiler as an error. The compiler also detects methods with override that don’t actually override any method in the superclass.

I would like to extend this cautious approach to protocols, forcing the developer to deliberately override an implementation that’s inherited from a protocol extension. This would prevent accidental overrides and force the user to proactively choose to implement a version of a protocol member that already exists in the protocol extension.

I envision this as using the same `override` keyword that’s used in class based inheritance but extend it to protocol inheritance:

protocol A {
    func foo()

extension A {
    func foo() { .. default implementation … }

type B: A {

    override required func foo () { … overrides implementation … }

I’d also like to bring up two related topics, although they probably should at some point move to their own thread if they have any legs:

Related topic 1: How should a consumer handle a situation where two unrelated protocols both require the same member and offer different default implementations. Can they specify which implementation to accept or somehow run both?

type B: A, C {
    override required func foo() { A.foo(); C.foo() }

Related topic 2: How can a consumer “inherit” the behavior of the default implementation (like calling super.foo() in classes) and then extend that behavior further. This is a bit similar to how the initialization chaining works. I’d like to be able to call A.foo() and then add custom follow-on behavior rather than entirely replacing the behavior.

type B: A {
    override required func foo() { A.foo(); … my custom behavior … }

cc’ing in Jordan who suggested a new thread on this and Doug, who has already expressed some objections so I want him to  have the opportunity to bring that discussion here.

— E

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